1. FIELD OF THE INVENTION
This invention relates to an oxygen concentration detector which detects oxygen concentration in a gas by using an oxygen concentration cell made of an oxygen-ion-conducting solid electrolyte.
2. DESCRIPTION OF THE PRIOR ART
To measure the oxygen concentration in exhaust gas from internal combustion engines, oxygen concentration detectors made of oxygen-ion-conducting solid electrolyte and based on the principle of an oxygen concentration cell have been used. A typical example of such oxygen concentration detectors uses yttria-added zirconia porcelain as the solid electrolyte with platinum electrodes mounted thereon. Conventionally, the oxygen concentration detectors are used to control the air-fuel ratio .lambda. at 1.0. However, the fuel efficiency is high under lean burn conditions wherein air content is higher than that at the air-fuel ratio .lambda. of 1.0, while the output efficiency is high under rich burn conditions wherein air content is lower than that at the air-fuel ratio .lambda. of 1.0. In order to control the air-fuel ratio .lambda. at a level which is different from 1.0 for achieving a high fuel efficiency or a high output efficiency, it is necessary to accurately measure the electromotive force and the temperature of the oxygen concentration cell. Oxygen concentration detectors capable of simultaneous measurement of the temperature and the electromotive force are known; for instance, detectors using temperature sensing elements for measuring the temperature of the solid electrolyte, detectors using coiled heater wires disposed in the solid electrolyte for heating them, and detectors using both the temperature sensing elements and the coiled heater wires for effecting both the heating and temperature measurement of the solid electrolyte.
The oxygen concentration detectors of the prior art have shortcomings in that the temperature of the solid electrolyte exposed to the exhaust gas from an internal combustion engine is not uniform and the temperature measurement at one point thereof does not provide accurate picture of the temperature of solid electrolyte; that when the temperature of the exhaust gas is changed the output of the temperature sensing element has a time lag in detecting the temperature change of the solid electrolyte so that the temperature of the solid electrolyte cannot be measured accurately; that when the temperature of the oxygen concentration detector is low, the catalytic ability of platinum is reduced while the electric resistance of the solid electrolyte is increased and the impedance of the oxygen concentration detector becomes high, and the reduced catalytic ability and the high impedance tend to render the oxygen concentration detector susceptible to adverse effects of noises or the like and to reduce the response speed thereof; and that the structure of the oxygen concentration detectors are complicated. Thus, the conventional oxygen concentration detectors for the measurement of both the electromotive force and the temperature of the solid electrolyte are not practicable.
It has been proposed to apply a direct current to the oxygen concentration cell so as to shift the rapid change point of the electromotive force of the cell from a level corresponding to the above-mentioned air-fuel ratio .lambda.=1.0 to another level. However, such proposal has a shortcoming in that accurate measurement of oxygen concentration cannot be obtained because the polarization becomes excessively large at low temperatures and the fluctuation of the air-fuel ratio .lambda. with temperature is large.